The present invention relates to an improved method of production of micronized material in a jet milling operation. More particularly, the present invention relates to the use of a humidified gas stream at a controlled temperature, pressure and relative humidity to achieve the maximum nonequilibrium moisture levels in the jet milling operation without the generation of a spontaneous water condensation shock.
Jet milling is a widely used technique, especially in the pharmaceutical industry for the production of fine particles through a micronization process. The development over the years of the many different milling technologies led to the appearance between the 1930's and 1940's of the first jet mills. During the period following World War II, jet milling technology was used for a variety of applications, including pesticides and pigments. The original principles of jet milling are feeding powder particles into the flat cylindrical milling chamber tangentially through a venturi system by pressurized air or nitrogen. The particles are accelerated in a spiral movement inside the milling chamber by a number of nozzles placed around the periphery of the chamber.
The micronizing effect takes place by the collision between the incoming particles and those already accelerated into the spiral path. While centrifugal force retains the larger particles at the periphery of the milling chamber, the smaller particles exit with the exhaust air from the center of the chamber. The particle size distribution is controlled by adjusting a number of parameters, two of the main ones being: pressure and feed rate.
In a jet milling operation, a supersonic nozzle with supply pressures of about 6 to 12 barg nitrogen entrains a feed gas containing material to be milled. The actual milling operation occurs downstream of the nozzle at close to atmospheric pressure, and has a time duration measured in milliseconds. The ultimate outlet temperature of the jet milling operation is typically at a relatively warm temperature (about room temperature). That is, the gas is introduced into the mill at about room temperature, and exits the mill at about room temperature. In between, the gas will change temperature significantly as it exits the supersonic nozzle (lower pressure and temperature) and is subsequently warmed by the energy released in the jet milling operation.
It is considered advantageous to perform the micronization process with humidified gas (typically air or nitrogen) to produce the best particles in terms of size, stability and other valuable properties. It is further considered advantageous to maximize the amount of water vapor present during the micronization process, without producing liquid condensate. The present inventor has discovered a method to maximize the amount of non-condensed water present in the gas stream participating in the micronization process.